This application claims priority of European Patent Application No. 01810417.4, filed on Apr. 27, 2001.
The invention relates to a drilling jig for the determination of the axis of a femur head prosthesis having a guide tube around whose longitudinal axis a sensing probe can rotate in a plane of rotation to sense a neck of the femur at the transition to the femur head and to centre the position of the longitudinal axis with respect to the neck of the femur by a displacement of the guide tube.
Such drilling jigs are used to drive a Kirschner nail through the femur head deep into the neck of the femur and to use this Kirschner head as a centring and alignment device for the resurfacing tools with which the femur head is resurfaced to the internal dimensions of a matching femur head prosthesis. A corresponding operative technique is described in xe2x80x9cBirmingham Hip Resurfacing Operative Technique According to Derek McMinnxe2x80x9d of Midland Medical Technologies (University of Birmingham Research Park, Birmingham, England). The drilling jigs used there show a bearing yoke whose lower bearing is supported on a collar pin which first has to be driven in by the operator in the direction of the neck of the femur from the lateral side beneath the trochanter major. A circular guide, which is aligned to the lower bearing and which receives a guide tube longitudinally displaceable therein, is located in the upper part of the bearing yoke. A rotatable sensing probe is attached to the guide tube and the spacing to the lower edge of the head of the femur can be checked with its inwardly directed sensing tip. The contact point can be centred by a repeated raising of the guide tube from the head of the femur and by putting it down in a new position by means of the rotation of the sensing probe and subsequently lightly driven in a little in order to define the axis direction for a Kirschner nail or another drill. If a Kirschner nail was used, the guide tube is first pulled off upwardly and the bearing yoke is subsequently withdrawn. The guide tube and the sensing probe can again be set onto the Kirschner nail for the visual control of the centring. This method has the disadvantage that, for the setting of the collar pin at a spacing beneath the trochanter major, a relatively large region must additionally be opened which is outside the actual operation region in which the head of the femur is resurfaced and a femur head prosthesis put on. A further difficulty lies in the fact that the effective neck angle of the neck of the femur should be taken into account in the selection of the driving in point for the collar pin.
The invention is intended to improve these circumstances. It has the object of providing an expedient drilling jig. This object is satisfied in accordance with the independent claim 1 in that the guide tube is pivotally mounted with a rounded head in a housing and can be fixed with first clamping elements at a designated pivot angle xcex1 of its longitudinal axis relative to the longitudinal axis of the housing, and in that the housing can be displaced in any direction transversely to its longitudinal axis with respect to a base part with a recess which is provided for the fastening to a femur head and can be fixed in a designated displacement position by second clamping elements.
The invention has the advantage that the angular position of the longitudinal axis of the guide tube relative to the support surface of the base part and the point of intersection of the longitudinal axis with the plane of the support surface can be freely chosen. It is sufficient if the operator makes a first resection area at the femur head as a base for the base part, with this area only having to be approximately perpendicular to the axis of the neck of the femur. As soon as the base part is fastened to the first resection area, for which it has, for example, anchoring pins at its lower side for driving in, the parallelism of the guide tube to the axis of the neck of the femur can first be checked and set from two different directions and subsequently the longitudinal axis of the guide tube can be displaced in the plane of the support surface until an optimum distribution of the cross-section of the neck of the femur is achieved during the rotation of the sensing probe.
Advantageous further developments of the invention result from the dependent claims 2 to 10.
The division into two independent clamping elements allows the pivot angle xcex1 and the position of the longitudinal axis of the guide tube to be set in succession and independently of one another. Undercut guides of the base part and the housing engage into a displacement part located between them. Since the guides intersect when viewed in the direction of the longitudinal axis of the housing, the housing moves like a cross slide relative to the base part. The base part and the displacement part have a recess when viewed in the direction of the longitudinal axis of the housing, inside which the lower end of the guide tube can be displaced. When a clamping nut is adjustable on an external thread of the housing in the direction of the longitudinal axis of the housing and radially projects further from the longitudinal axis, torques are produced on the guides of the displacement part which result in an elastic deformation of the displacement part when this is deliberately weakened by flexion spring sections. This effect is amplified more when the upwardly directed guide surface of the guides of the housing has a slightly convex curvature. The guides of the housing and the base part can be simultaneously blocked in this way with a clamping nut.
The head of the guide tube is spherically rounded and sits on a bearing surface of the housing. The oblique shoulder of a clamping sleeve in the housing which is adjustable in its thread presses the head into its bearing surface and thus blocks it in a designated angular position.
It has quite generally been found in the positioning of a guide tube with a sensing probe which can rotate on it, that a sensing and centring of the neck of the femur directly beneath the femur head result in a good distribution of the harder cortex layer onto the femur head prosthesis if the sensing radius ri from the axis of rotation to the tip of the sensing probe is smaller than the inner radius Ri of the femur head prosthesis at its lower edge by an amount of 1.2 to 2.5 millimeters. This makes necessary a whole kit of femur head prostheses with associated sensing probes. With a sufficiently fine gradation, the sensing radius can be chosen smaller than the inner radius of the femur head prosthesis at its lower edge by an amount of between 1.5 to 1.9 millimeters. The ideal inner radius of the femur head prosthesis at its lower edge for the associated sensing probe, which can just rotate in the centred state, would then be 1.5 to 1.9 millimeters larger than the radial spacing of the sensing tip.